Effects of an Electric Current and Domestic Circuits

Chapter 24

Heating Effect of Electric Circuits• It was James Watt who experimentally

investigated the effects of the heat, W, from a current carrying wire. It was found that the following equation may be used to find W:

• where W = amount of heat energy given I = current through the wireR = resistance of the conductor

t = time that the current flows for

RtIW 2

• Joule’s law states that the rate at which heat is produced in a conductor is directly proportional to the square of the current provided its resistance is constant:

• By dividing both sides of the heat effect equation by ‘t’ we find:

2IP

RIP 2

The Chemical Effect of an Electric Current

• An electric current may cause a chemical reaction when passed through a liquid, known as electrolysis.

• The liquid in which the current is passed is called the electrolyte, the plates that are in the electrolyte are called electrodes, the positive electrode is known as an anode, the negative electrode is the cathode. The container, electrodes and electrolyte are known as a voltameter.

• Electrodes that are involved in the chemical reaction are called active electrodes, those that do not are called inactive electrodes.

• Examples of an electrolyte would be water with an acid, base or salt in it (i.e. a solution), or an ionic compound in it’s molten state.

• An ion is an atom or molecule that has gained or lost one or more electrons.

• The charge carriers in an electrolyte are positive and negative ions.

Applications of the Chemical Effect• Electroplating, covering one metal with a

thin plating of another.• Extraction of minerals from their ores.• In electrolyte capacitors electrolysis is used• To purify metals.

Relationship between current and voltage for different conductors

A metallic conductor • Assuming we have constant

temperature, the resistance of a conductor will not change as current increases, so we get a straight line graph through the origin, obeying Ohm’s law.

• Charge carriers in a metal are electrons.

A filament bulb As the voltage across the filament increases, so does the current and in turn the temperature. Since resistance α current, resistance increases. So as the bulb gets hotter a given increase in V doesn’t produce an increase of I when it was colder.Charge carriers in a filament bulb are negative electrons.

A semi conductor, e.g. a thermistor

As the p.d. across the semiconductor is increased, the current increases, and in so doing the semiconductor gets hotter. This produces more holes and electrons for conduction and the resistance drops. Thus a further increase in V produces a larger increase of I when it was cold.Charge carriers are negative electrons and positive holes.

Electrolytes/ionic solutions

As p.d. increases so will current. Active electrode takes part in the chemical reaction-also obey Ohm’s law and the graph is linear through the origin. If the electrodes are inactive, the voltameter behaves like a cell and has an emf that must be overcome before current will flow.Charge carriers are positive and negative ions.

Active electrodes

Inactive electrodes

A Gas An example would be a discharge tube. In region OA the positive ions in the tube are attracted to the negative electrode and the electrons move towards the positive electrode once a p.d. is applied, as number of ions crossing the tube increases so does the current. In region AB all the ions in the tube cross without recombination so no increases in current. Voltage increases to a stage where collision between fast moving ions and electrons produces more ions, corresponding to region BC.Charge carriers are positive ions, negative electrons and a few negative ions.

E.g. neon lamps.

A Vacuum

No charge carriers in a vacuum, however if the cathode is heated sufficiently electrons will be produced by thermionic emission. A certain voltage is reached where all the electrons from the cathode are carried across the tube and the curve flattens out.

Domestic Electric Circuits• Appliances that take a large current , such as an

electric cooker, electric shower or immersion heater have a separate live and neutral wire coming from the distribution box. Such a circuit is called a radial circuit.

• In a ring circuit, the live terminals of each socket are connected together. Power is thus fed along both sides of the ring to each socket. The neutrals are also connected together and connected back to the neutral at the distribution box.

• In a plug the live wire is brown, neutral is blue and earth is green/yellow.

• Bonding is when metal taps, metal water tanks, etc. in a house are earthed in case they ever come into contact with a live wire, thus nobody is electrocuted.

• Earthing is where an appliance is earthed so that if a fault develops where the live wire came in contact with the outer metal casing, for example, nobody would be electrocuted if they touched the casing.

• A fuse is a piece of wire within a ceramic casing that is placed in series with the live wire of an electrical appliance, normally in the plug. If a fault develops within the appliance where it is drawing too much current the wire within the fuse would be unable to maintain this extra current and thus melt, thus breaking the flow of electricity and taking away the possibility of electrocution.

• Miniature circuit breakers(MCBs) are used sometimes instead of fuses in the distribution box. They contain a bimetal strip and an electromagnet. When the current is larger than the preset value, two contacts are separated and thus breaking the flow of electricity. Bimetal strip is used to trip small currents, while the electromagnet is used for larger currents.

• Residual current devices(RCDs) operate by detecting a difference between the current in the live and the neutral-which could arise if somebody came in contact with a live wire. When the difference between the two reaches a preset value ( normally 30mA ), the RCD trips very quickly and cutting off electricity, whereas the fuse and MCB would take longer.

The Kilo-watt Hour• The kilo-watt hour is the amount of energy

used by a 1000 W appliance in one hour. Remembering that:

1000 W=1000 joules per second and

energy=power x time.